(a) Field of the Invention
This invention relates to diopter adjustable eyepieces used for view-finders of single-lens reflex cameras or the like.
(b) Description of the Prior Art
Generally, in order to adjust the diopter of an eyepiece, the eyepiece may be moved forward and rearward with respect to the object. However, in case the diopter is to be adjusted by this system, the movement .DELTA. per diopter of the eyepiece will become so large that, in case this system is incorporated in an optical instrument, the optical instrument will become so large as to be inconvenient. If f.sub.e is the focal length of the eyepiece, the movement .DELTA. will be given by the following formula: EQU .DELTA.=(f.sub.e.sup.2 /1000)
Therefore, in order to make .DELTA. small, as shown in FIG. 1, it will be considered to form the eyepiece of a concave lens and convex lens, to fix either one of the concave lens and convex lens and to make the other movable. For example, if the concave lens is fixed and the convex lens is made movable, if the focal length of the convex lens is f.sub.p, the movement .DELTA..sub.p of the convex lens will be given by the following formula: EQU .DELTA..sub.p =(f.sub.p.sup.2 /1000)
If the composite focal length of an eyepiece consisting of a concave lens and convex lens as in this example is equal to the focal length f.sub.e of an eyepiece consisting of only a convex lens, as f.sub.p &lt;f.sub.e, .DELTA..sub.p &lt;.DELTA. will be made. Therefore, the movement of the lens at the time of adjusting the diopter will be smaller in the case that the eyepiece is formed of two lenses than in the case that the eyepiece is formed of one lens.
However, this system shown in FIG. 1 of moving the convex lens in the eyepiece has such defect as in the following. If the focal length of the concave lens is f.sub.n and the distance between the concave lens and convex lens is d, in order that f.sub.e may be of a predetermined value, it will be necessary to determine f.sub.n by the following formula: ##EQU1##
If the distance from the object to the concave lens is S, in case the diopter of the eye is zero, that is, the virtual image formed by the eyepiece is infinitely far, in order to make S&lt;0, d&lt;f.sub.p will have to be made. Further, according to the formula (1), if f.sub.p is made small, .vertline.f.sub.n .vertline. will be also become small. That is to say, if the power of the convex lens is made strong in order to make the movement .DELTA..sub.p small, the power of the concave lens will have to be also made strong. In such case, even if the eyepiece is so designed that the aberrations will be canceled at a certain diopter, if the distance d is changed to adjust the diopter, the cancelation of the aberrations of the convex lens and the aberrations of the concave lens will be imperfect and aberrations will occur in the entire system. If the power of the convex lens is weakened in order to make the variations of the aberrations small, in the case of adjusting the diopter in a wide range, the movement of the convex lens will become large and the optical instrument will become so large in the total length as to be inconvenient to use.